The word rumpus can refer to many things such as your ass, toshi, butt, booty, backside, etc. The word rumpus comes from the word ramp which can sound like rump which can mean ass, toshi, butt, booty, backside, etc. Rumpus can also refer to a group of asses, toshis, butts, bootys, backsides, etc. Which is in the plural form. It can also mean loud noise or mess.

"Girl, you have a fine rumpus."
"Girl, clean up this rumpus."
"What is this loud rumpus."

Contrary to popular belief, rumpus doesn't mean your ass.
A rumpus is an activity that typically takes place after dark where a lot of people build a huge bonfire. They then paint their faces and/or arms and chest. You may also strip down to your skivvies but it's not necessary. After you're all dressed for the rumpus you dance around the fire and yell and sing. The object is to let your primal nature take over. This activity is mostly participated in by hippies. A rumpus took place in the book Where the Wild Things Are.

"Hey Olivia, you coming to the rumpus tonight?"
"Hell yea! I love to rumpus!"